For example, if a part to be processed is subjected to padding by means of laser beams (laser cladding) to form a cladding layer, it is customary that a powdery cladding material be applied to or deposited on the surface of a base material and that the cladding material be irradiated with laser beams. The laser beams that have been radiated are multiplicatively reflected in the cladding material and are thereby absorbed while being attenuated, thus heating the cladding material and welding it to the base material. It is to be noted herein that the distribution of energy of generally employed laser beams with which a part to be processed is irradiated is either defocused and adapted for Gaussian mode (single mode) or adapted for multiple mode so as to ensure that the laser beams exhibit an intensity of a predetermined level continuously over a certain width (although it is in fact difficult to maintain the intensity of the laser beams at the predetermined level).
As a more concrete example of performing padding by means of such laser beams, it is known to supply a wear-resistant and corrosion-resistant cladding material that is different from a material for an intake or exhaust valve body of an engine or its valve seat portion (base material) to the surface of the base material through a nozzle or the like with a view to enhancing the wear resistance and corrosion resistance of the valve body or the valve seat portion, irradiate the cladding material with laser beams, weld the cladding material to the valve body or the valve seat portion (base material), and clad it with a cladding layer.
One of the arts applicable to such an example is disclosed in Japanese Patent Application Laid-Open No. HEI 9-239574 as a method of manufacturing an engine valve. This publication mentions “that a method of manufacturing an engine valve (by cladding it with a cladding layer through radiation of laser beams) requires uniformly heating a powdery material for padding (cladding material) over the entire region in the radial direction of a valve body and that if a base material is for example heated partially and excessively, the phenomenon of dilution, namely, penetration of constituents of the base material through a padding material occurs as a result of fusion of the base material and makes it impossible for the padding material (cladding material) to exhibit its inherent properties such as wear resistance or causes inconveniences such as incomplete welding, underfill, fusion sag, and the like of the padding material” (see paragraph 0003 of this publication).
As a problem to be solved, this publication mentions “that the base material of the valve body at the beginning of padding is at a room temperature, that the heat of laser beams is conveyed to the base material at a position to be padded later as the operation of padding based on radiation of laser beams proceeds, that the later the base material is padded at a certain position, the higher the temperature of the base material at the position becomes, and that the temperature of the base material almost at the end of padding is higher than the temperature of the base material at the beginning of padding” (see paragraph 0005 of this publication). As a means for solving the problem, Japanese Patent Application Laid-Open No. HEI 9-239574 discloses “a method of manufacturing an engine valve wherein a groove portion is formed in a valve body in its region that serves as a valve face and wherein laser beams are displaced relatively in the circumferential direction of the groove portion so as to perform padding while the powdery material for padding that has been supplied to the groove portion is being irradiated on the surface of its powder layer with the laser beams, characterized in that distribution of output energy of the laser beams is controlled such that a marginal portion at the time of each padding operation is maintained at a substantially constant temperature from the beginning of padding to the end of padding” and “a method of manufacturing an engine valve wherein a groove portion is formed in a valve body in its region that serves as a valve face and wherein laser beams are displaced relatively in the circumferential direction of the groove portion so as to perform padding while the powdery material for padding that has been supplied to the groove portion is being irradiated on the surface of its powder layer with the laser beams, characterized in that the surface of a powder layer is irradiated with the laser beams whose spot diameter has been narrowed down to a value smaller than the width of the groove portion, that the laser beams having the spot diameter thus narrowed down are oscillated in the radial direction of the valve, and that the width of oscillation is reduced after padding has been started”.
Furthermore, as an effect of the construction as described above, this publication mentions “that since the marginal portion of each padding position can be maintained at a suitable temperature from the beginning of padding to the end of padding, it is possible to inhibit the padding material from decreasing in wear resistance or corrosion resistance or from undergoing incomplete welding or underfill due to dilution of the base material in the marginal portion that has a low heat capacity, perform padding uniformly and with good quality over the entire region, and provide a high-quality engine valve” (see paragraph 0035 of this publication).
As is also shown in FIG. 12, a device for performing padding by means of laser beams thus narrowed down (adapted for Gaussian mode) comprises a parabolic mirror 31 for condensing oscillated laser beams, an oscillating mirror 32 for oscillating laser beams 30 that have been condensed and adapted for Gaussian mode in accordance with a width W of a cladding material 1 that has been deposited on the surface of a base material 5 as a part to be processed, and a driving means (not shown) for driving the oscillating mirror 32 in a fluctuating and reciprocating manner, such as a galvanometer or the like. Furthermore, in general, a nozzle 7 for supplying shielding gas to the cladding material 1 around its region irradiated with laser beams is provided.
In the case where a valve seat or the like is thus subjected to a predetermined processing such as padding or the like by means of the laser beams 30 that have been adapted for Gaussian mode, it has been customary to displace the laser beams 30 relatively in the longitudinal direction (or in the circumferential direction) of the part 1 to be processed while oscillating them in the direction of the width W (or in the radial direction) of the part 1 to be processed.
However, as described above, in the case where padding is performed by means of laser beams, laser beams that have been radiated are multiplicatively reflected by a cladding material and absorbed while being attenuated, whereby the cladding material is heated and welded to a base material. Hence, if the cladding material that has been deposited with different thicknesses is irradiated with laser beams of the same intensity, it exhibits a higher degree of multiplicative reflection of laser beams and thus absorbs a greater amount of the radiated laser beams in its region that has been deposited to a great thickness. Thus, as compared with the region that has been deposited to a small thickness, the region that has been deposited to a great thickness exhibits a higher degree of distribution of heat input and is more likely to be heated. On the other hand, the cladding material that has been supplied to and deposited on the surface of the base material by means of a nozzle or the like is generally thick in its widthwise central portion and thin in its widthwise marginal portions. Accordingly, in the case of irradiating the cladding material that has been supplied to the surface of the base material from the nozzle with laser beams of a uniform intensity, the cladding material is heated excessively in its widthwise central portion that has been deposited to a great thickness so that dilution of the base material tends to occur, and the cladding material is heated insufficiently in its widthwise marginal portions that have been deposited to a small thickness so that incomplete welding tends to occur.
The conventional art disclosed in the aforementioned Japanese Patent Application Laid-Open No. HEI 9-239574 takes into account the fact that the heat of laser beams during the operation of padding is conveyed to the base material at its position to be padded later so that the base material rises in temperature, and is designed to adjust the distribution of output energy in the processing (longitudinal) direction of the laser beams such that the marginal portion of the engine valve to be padded is maintained at a substantially constant temperature from the beginning of padding to the end of padding. This conventional art does not consider the difference in distribution of heat input in the direction of the width of the cladding material.
Further, as described above, this publication mentions that the art disclosed therein requires uniformly heating the entire region in the direction of the width of the cladding material. For this purpose, laser beams are oscillated with respect to the cladding material in the radial direction (the direction of the width) of the valve. Therefore, a cladding layer 1′ is formed with an undulant surface or meandrous marginal portions 1e′. The problem is that the cladding layer 1′ cannot be formed smoothly. In the case where a valve body or a valve seat is subjected to laser cladding, the surface of a cladding material that has been formed to ensure sealing properties during abutment of the valve body on the valve seat is ground or subjected to other processings. However, if the cladding layer 1′ is formed with an undulant surface or meandrous marginal portions, the amount of the cladding layer 1′ to be ground or subjected to other processings is increased. The problems are that the method becomes time-consuming and laborious and that a certain amount of the cladding material 1 is wasted.
Furthermore, in the case where the laser beams 30 that have been adapted for Gaussian mode are oscillated within a predetermined width of the part to be processed, there is a problem of dependency of the distribution of heat input on the diameter of the condensed laser beams 30. That is, as shown in FIG. 13, it is assumed for example that the part to be processed has the width W of 5 mm and that the condensed beams 30 have a diameter of 3 mm, 2 mm or 1 mm. If the condensed beams 30 have a diameter of 3 mm, they overlap with one another at the center in the direction of the width W of the part 1 to be processed, so that the distribution of heat input is biased toward the center in the direction of the width W of the part 1 to be processed. If the condensed beams 30 have a diameter of 2 mm, the part 1 to be processed is irradiated with the beams in its width (W)-wise marginal portions over a longer period than in its width (W)-wise central portion, so that more heat is inputted to the width (W)-wise marginal portions of the part 1 to be processed than to the width (W)-wise central portion of the part 1 to be processed. If the condensed beams 30 have a diameter of 1 mm, the part 1 to be processed is irradiated with the beams in its width (W)-wise marginal portions over a much longer period than in its width (W)-wise central portion, so that the distribution of heat input is biased toward the width (W)-wise marginal portions of the part 1 to be processed and that the amount of heat inputted to the width (W)-wise central portion of the part 1 to be processed is insufficient. That is, the problem is that the distribution of heat input cannot be stabilized. In the case of padding, if the distribution of heat input is biased toward a certain portion, the phenomenon of dilution, namely, penetration of the base material 5 through the cladding material 1′ occurs, and it becomes impossible to perform the function of enhancing wear resistance and the like of the valve seat or the like. That is, the problem is that the part to be processed is heated excessively and can no longer be processed as originally intended. Further, in the case of padding, if the intensity of laser beams is lowered with a view to preventing the part to be processed from being heated excessively, the cladding material cannot be welded to the base material. That is, the problem is that the amount of heat inputted to the part to be processed becomes insufficient so that the part to be processed can no longer be processed as originally intended.
In addition, since the device for oscillating laser beams is designed to drive the oscillating mirror 32 in a fluctuating and reciprocating manner, it is complicated in structure and control logics. Another problem is that the maintenance operations for removing dirt from the oscillating mirror 32 etc. are laborious.
On the other hand, in the case where the part to be processed is irradiated with laser beams that have been adapted for multiple mode, in order to maintain the intensity of the laser beams at a predetermined level continuously in the width direction, it is necessary to average the intensity of the laser beams at the predetermined level in the width direction. The problem is that the laser beams decrease in energy efficiency. Another problem is that it is difficult to actually maintain the intensity of the laser beams at the predetermined level. In addition, as is apparent from a comparative example shown on the right side of FIG. 6(a), even in the case of radiating laser beams that have been adapted for multiple mode, the cladding material 1 is heated excessively in its width (W)-wise central portion that has been deposited to a great thickness as described above, so that a corresponding portion of the base material 5 is heated up to the extent of causing dilution (see a comparative example shown in FIG. 6(b)). Further, the cladding material 1 is heated insufficiently in its width (W)-wise marginal portions that have been deposited to a small thickness, so that incomplete welding is caused. As a result, as is apparent from a comparative example shown in FIG. 6(c), the apex of the cladding layer 1′ that has been formed is bulgy and biased toward a location which is heated excessively and to which a great amount of heat is inputted. Thus, in the case of padding a valve seat or the like, the cladding layer 1′ is not smooth enough to be welded to the base material 5. The problems are that the amount of the cladding layer 1′ to be ground later is increased and that the valve seat becomes incapable of performing its inherent function.
Such problems ascribable to the dependency of the distribution of heat input on the position in the direction of the width of the part to be processed occur not only in the case where padding such as laser cladding or the like is performed but also in the case where laser beam welding, laser beam hardening, or the like is performed.
The present invention has been made as a solution to the aforementioned problems. It is an object of the present invention to provide a method of laser beam machining which has a simple construction and which is capable of suitably processing a part to be processed by means of laser beams.